The increasing demand for fuels alternative to oil derivatives is directing towards processes for production of bio fuels. The production of ethanol and biogas through the digestion process starting from complex organic matrices of a biological nature (biomasses) coming from agricultural production lines (for example residual biomasses of food or industrial crops and agro-industrial or urban waste) represents a promising way to produce renewable energy and is one of the preferred ways of disposing of organic residues and waste. A considerable portion of biomasses (about 75%) is represented by the cell walls of plants. These consist of a heterogeneous matrix composed of polymeric carbohydrates associated to other components, such as lignin and proteins. Wall polysaccharides produce simple sugars by degradation (saccharification) which can then be used by the micro organisms present in aerobic and anaerobic fermentation for the production of bioethanol and biogas, respectively. Enzymatic hydrolysis is currently regarded as the most promising technology and with the lowest environmental impact for “saccharification”, that is, the conversion of raw plant material into fermentable sugars. The limiting factor in this process is the natural resistance of cell walls to enzymatic degradation. Pre-treatment technologies currently available for making biomasses sensitive to hydrolytic enzymes that degrade the single components of the plant cell wall are expensive and in some cases require, along with mechanical break-up processes, the use of toxic and/or polluting substances, such as acids, peroxides and ammonia.
The availability of more easily degradable plant material would considerably improve the use of biomasses and would reduce the need for expensive pre-treatments with a strong environmental impact. Since the accessibility of the cellulose component to the degrading enzymes is prevented by the presence of pectins, hemicelluloses and lignin, qualitative and quantitative modifications of these components in the plant walls may improve the efficiency of saccharification.
The technical problem the invention intends to solve is to decrease the recalcitrance of cell walls found in lignocellulosic biomasses to the enzymatic hydrolysis by cellulases and other degrading enzymes.
The authors have set up a process that reduces, in the cell wall of a plant, the presence of de-esterified homogalacturonan, a component of plant pectins. The reduction is achieved by expression of a hexogen polygalacturonase, for example fungal polygalacturonase (PG), and/or overexpression of a pectin methylesterases inhibitor (PMEI). The process lead to an increase in the efficacy of cellulose enzymatic hydrolysis.
The biomasses containing the pectin thus modified do not need expensive and polluting pre-treatments and allow improving the saccharification process.